The present invention relates generally to gas turbine combustors and more specifically to an apparatus and method for reducing carbon monoxide emissions from gas turbine combustors.
In recent years government officials have passed more restrictive regulations regarding powerplant emissions, especially those for oxides of nitrogen (NOx) and carbon monoxide (CO). Each of these emissions are well known to contribute to air pollution and regulators continue to set lower levels of acceptable emissions. There are various means to comply with these lower emissions requirements, which vary depending on the powerplant location. Such means include passing the exhaust gases through a catalyst, which serves to transform the carbon monoxide and remaining hydrocarbons into water and carbon dioxide, utilizing lower flame temperature combustors, or limiting the amount of operating time of the powerplant. The latter is the most unfavorable option as it limits the amount of revenue that can be generated. However, the other technologies such as a catalyst and lower flame temperature combustors can be expensive as well.
Complying with environmental requirements is especially a concern when the powerplant is operating at a load point other than its preferred condition. Powerplants are designed to operate most efficiently at the “full-load” condition, that is when they are generating the most power possible, and it is at this condition that they are designed to produce the lowest emissions. However, there are many times when power demand is lower and it is more desirable to operate at a lower power setting, such that only the power demanded is actually supplied, thereby saving on fuel costs. It has been determined that when powerplants operate at conditions other than their most efficient, or design point, emission levels can go out of compliance with local regulations. This is especially true for NOx and CO and the present invention described herein addresses CO emissions reductions. Carbon monoxide from gas turbine combustion systems can typically be caused by a number of factors including inadequate burning rates, inadequate mixing of fuel and air prior to combustion, or quenching of the combustion products in surrounding cooling air. When combustion gases migrate towards a region containing cooler air, the temperature of this air, which is cooler than that of the hot combustion gases, prevents any further chemical reactions from occurring and CO will remain in the exhaust gases.
In order for powerplants to run at lower load conditions, where emission levels can be higher, it is necessary to be able to control the amount of emissions that will result when the combustion system is not operating at its preferred design point. A condition at which higher CO emissions are especially prevalent is at lower power settings. At these lower power settings, the combustion systems are operating at a lower fuel flow and often times burning in a different region than that of the full power condition that may not be as efficient. Therefore, in order to operate a powerplant with reduced CO emissions throughout its operating envelope, it is necessary for the combustion system to be able to provide adequate mixing such that the CO is not quenched and the combustion reactions are completed.
The present invention seeks to overcome the shortcomings of the prior art by providing an apparatus and method of reducing carbon monoxide emissions for a gas turbine combustion system.